Various types of fluid pressure sensing devices are currently available for use as safety devices in the monitoring and sensing of process pressures such as at oil and gas wells, petrochemical sources, and associated production and processing installations. The U.S. Government has specified certain performance and safety requirements for such pressure sensors, known in the trade as "pressure pilots". The required sensors or pilots must be highly accurate and extremely reliable in order to properly protect production equipment, operating personnel and the environment at oil and gas drilling and processing plants on off-shore installations.
Thus, presently available pressure sensors or pressure pilots are designed to automatically activate or trip safety shutdown systems when the sensed process pressure either extends above a previously set high pressure point or extends below a low pressure set point. When the process pressure returns to the desired safe range within the high and low pressure set point limits, the pressure sensor should be capable of automatic reset.
Therefore, it is desirable that a pressure sensor be capable of accurate set point tripping under repeated conditions (this function being known as "set point repeatability"), and once tripped should have the capability to reset within a fairly narrow zone (commonly known as "trip-to-reset"). Thus, it is desired that the tripping and resetting operations be accomplished very rapidly so as to provide a snap-acting operation in pressure ranges around the set point. In addition, it is desired that such pressure sensors be compact, protected from the environment, capable of installation in a variety of conditions, capable of being repaired in the field, and capable of providing high-only, low-only, or high and low pressure set points or pressure limits.
Many if not all presently available pressure pilots do not have all of the desired features. Certain pressure pilots are simple in construction and of rugged design as well as field repairable, but provide only a single high or low pressure set point capability with poor set point repeatability as well as a large set point trip-to-reset zone. Other available pressure sensor units of more complicated and therefore higher priced construction provide both high and low pressure set point capability, but may be highly sensitive to vibrations. This may result in poor set point repeatability and an excessive trip-to-reset zone. In addition, many of these units undesirably provide interactive set points; are not field repairable; and do not permit flexible mounting installations.